Perfluoroelastomers (elastomeric perfluoropolymers) are polymeric materials which exhibit outstanding high chemical resistance and temperature tolerance and accordingly are particularly adapted for many industrial uses in which elevated temperatures and/or corrosive chemicals are encountered. The outstanding properties of perfluoropolymers are largely attributable to the stability and inertness of the copolymerized perfluorinated monomer units which make up the major portion of the polymer backbone, e.g., tetrafluoroethylene and perfluoro(alkyl vinyl)ethers. In order to achieve good elastomeric properties, the perfluoropolymers must be crosslinked. To this end, a small percentage of a monomer having a functional group is copolymerized with the perfluorinated monomer units. Monomers containing nitrile functional groups, such as perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred. Such compositions are described in U.S. Pat. Nos. 4,281,092 and 4,394,489; and in International Application WO 95/22575.
The fluorinated polymers and particularly perfluorinated elastomers have unique thermal and chemical resistance properties. Preparation of these fluoroelastomers from fluoropolymer precursors (sometimes referred to as “raw gums” or “gum”), however, can be difficult. The fluoropolymer precursors and compositions containing the fluoropolymer precursors may be incompatible with processing and curing additives. In addition to incompatibility, certain curing additives are disposed to undesirable homopolymerization, which can lead to processing difficulties in preparing fluorinated elastomers.
The cured or cross-linked fluoroelastomers are tolerant to high temperatures and aggressive chemical environments and therefore are particularly useful as seals, gaskets, and molded parts in systems that are exposed to elevated temperatures and/or corrosive materials. For sealing applications that require resistance to the most extreme conditions, perfluorinated elastomers are used. Such parts are used in applications such as automotive, chemical processing, semiconductor, aerospace, and petroleum industries, among others.
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture under pressure, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly used curing agents for fluoroelastomers include difunctional nucleophilic reactants, such as polyhydroxy compounds. Alternatively, peroxidic curing systems containing organic peroxides and unsaturated coagents, such as polyfunctional isocyanurates, may be employed.
Many fluoroelastomers often include a functional group to facilitate cure in the presence of a curative or catalyst. One class of useful functional group used in perfluoroelastomers includes nitrile group-containing monomers, for which organotin catalysts have been used as curing components. However, such catalysts can leave undesirable extractable metal residues in the cured product and are undesirable for environmental reasons. Ammonia-generating compounds have also been used as a cure system component in fluoroelastomers, but these cure systems lack the desired level of rheological control during processing. In addition, most known fluoroelastomers are cured into colored or opaque materials. A few fluoroelastomers or perfluoroelastomers have been described as colorless and/or transparent, yet can be cloudy or milky in appearance, and have rather high compression set when made into seals, even when measured at relatively low temperatures (up to 200° C.).
Copolymers containing 50-75 Mole % of TFE links, 49.8-25.0% of PAVE links and 0.2-5 Mole % of perfluoroalkyl vinyl ether links having the following chemical structures, have been described in U.S. Pat. No. 5,565,512:
where n is 2 to 4, or
where m is 0 to 4.
The above perfluoroelastomer compositions include an ammonium salt of an organic or inorganic acid as a curing agent. The content of the curing agent is 0.2 to 5 parts by weight per 100 parts by weight parts of the perfluoroelastomer. The above compositions can include a filler (for example, Carbon Black), pigments, plasticizers and other special fillers.
Cured formulations of the above composition have high strength of around 17.8-22.2 Mpa, light gray color or transparent if not filled with Carbon Black. The above compositions have poor thermal resistance in stress conditions. Even at 200° C. for 70 hours the compression set is 37-49%. Also they are not stable to strong nitric acid (swelling in 60% HNO3 for 70 hours at 80° C. is +0.7).
U.S. patent application No. 2002/0061977 copolymer compositions containing 62% TFE, 36.8 mole % PAVE and 1.2 mole % perfluorovinyl ether of structure A or B (as shown above), including 0.05-10 weight parts of a curing agent of the structure R′C(OR)2═NH and its salts, where R′ and R are alkyl, aryl, aralkyl, alkenyl per 100 parst by weight of copolymer. The composition can also contain carbon, fluoropolymer fillers, plasticizers and other additives. Vulcanized compositions without carbon or colorant have no significant color change and have good physical and mechanical properties. According to the authors of this patent application compression set at 300° C. for 22 hours is 15.3%, but at 330° C. for 24 hours is 50-60%. Additionally, the strength of the unfilled vulcanized compositions is 5 to 9 Mpa.
Russian Patent No. 2,137,781 describes perfluoroelastomeric compositions based on a copolymer containing 42-70 mole % of TFE links, 25-55 mole % PAVE links, 1-4 mole % perfluorovinyl ether of structure A or B, also include 1 part by weight per 100 parts by weight parts of the copolymer, of 4,4′-[2,2,2-trifluoro-1-(trifluoromethyl)-ethyl-idene]bis(2-aminophenol) (CAS registry number 83558-87-6) referred to as BOAP in this patent as a curing agent. The composition can also contain a filler, for example, carbon black. Vulcanized compositions based on the above copolymer are well processed on conventional equipment and have good physical and mechanical properties. The disadvantage of such composition is visible color even in the unfilled state. According to the authors of the patent, compression set at 275° C. for 24 hours is 12-15% and at 300° C. is 26-51%. Also, the above compositions have poor thermal stability under stress conditions and poor stability in strong nitric acid.
Accordingly, there remains a need in the art for improved curing agents capable of more easily dispersing in and more quickly curing perfluoroelastomers, particularly cyano curable perfluoroelastomers. There is further a need in the art for a cure accelerator for perfluoroelastomer curatives which accelerate the cure rate of and maintain the beneficial properties of perfluoroelastomers. The shortcomings of the prior art agents noted above may be overcome by employing derivatives thereof in accordance with the present invention.